Field of the Invention
This invention relates to a method of making phosphor screens for cathode ray tubes. The invention is particularly directed to the so-called slurry-direct photographic process for making mosaic type screens having a systematically arrayed multiplicity of elemental phosphor areas such as dots or stripes. Such screens having three systematic arrays, each of a different color emitting phosphor may be used in apertured shadow mask type cathode ray tubes.
In making phosphor screens by this process, a slurry which comprises phosphor particles, a polymeric binder such as polyvinyl alcohol (PVA), a dichromate photosensitizer such as ammonium dichromate and potassium dichromate, and a suspension medium such as water. A quantity of the slurry is deposited near the center of a cathode ray tube face plate panel and is spread evenly thereover by spinning and tilting the panel according to a selected schedule. After the slurry has been spread over the face plate panel to provide a layer of desired thickness, it is dried. The phosphor coating on the face plate panel is then exposed to a desired pattern, e.g., dots or stripes, of light rays through a suitable apertured mask negative, which may eventually be incorporated as part of the finished cathode ray tube. The exposed layer is then developed by washing it to remove the unexposed phosphor areas, thus leaving the desired array of elemental phosphor areas adhered to the panel. The process is repeated for each of a plurality of desired phosphors, such as red emitting, green emitting and blue emitting to produce the finished screen.
For a detailed description of the photographic process refer to U.S. Pat. No. 3,406,068 issued to H. B. Law, Oct. 15, 1968.
It has been observed that when working with red emitting phosphor of the rare-earth oxide type, e.g., yttrium oxide and gadolinium oxide, that the phosphors are very slightly soluble in water. Due to this solubility a hydrolysis reaction occurs forming complexes of the cation of the rare-earth oxide phosphor and chromate-dichromate anions of the dichromate sensitizer thus producing degradation of the oxide phosphor, a change in body color, and rendering the slurry solution insensitive to radiation excitation. The period of time for which the hydrolysis takes place is dependent in part on the type of phosphor used. The gadolinium oxide exhibits a rapid deterioration whereas the yttrium oxide exhibits hydrolysis over a longer period of time.